The invention relates to an apparatus for forming a stack of sheets comprising a brake means placed at (in terms of sheet motion) the leading edge of the stack and downstream from a sheet transport means, the brake means having at least one brake roller driven at a lower peripheral speed than the speed of transport of the sheets and a timed sheet nip member adapted to engage a sheet and move it towards the brake roller.
An apparatus designed on these lines is to be seen in the German Pat. No. 3,007,435 and in it the brake means is made up of brake rollers which are driven at a speed that is lower than the sheet transport speed and against which the respective sheet to be retarded is pressed by an eccentric member running at the sheet transport speed. It this case it is therefore possible for the sheet to be braked to be propelled by the eccentric member, running at a higher speed than the respective brake roller over the respective brake roller. Therefore there is the danger of the sheet, while still moving at a relatively high speed, violently colliding with a stop mounted at the trailing edge of the stack so that the leading edge of the sheet will be damaged. A further danger with the known system is that the respective sheet will be differentially braked at the different braking rollers dependent on whether the sheet speed is nearer the speed of the brake rollers or nearer to the speed of the eccentric member cooperating therewith. The consequence is that the sheet will veer about an axis perpendicular to the plane of transport and this may cause a stoppage, something that will be the more likely at high sheet velocities. Although others have made attempts to remedy or improve such systems by replacing the eccentric sheet nip members by nozzles to press the respective sheet, that is to be slowed, down against the brake rollers, this has not proved to be a remedy to the shortcomings, since, more especially in the case of stiff material, the pressing forces due to the nozzles are insufficient to cause an even braking action. Furthermore, in such an apparatus the air coming from the nozzles is prone to cause undesired side-effects.
However, quite apart from the effectiveness or ineffectiveness of the retarding action of the arrangement in accordance with the German Pat. No. 3,007,435, there is in any case the difficulty that the sheets to be deposited in a stack are not reliably straightened or flattened. Although a flattening effect is produced by the inertial forces coming into effect during the braking operation, because of the comparatively small mass such forces have not proved to be great enough to cause effective flattening. There is furthermore the danger of the leading edge of the sheet moving downwards onto the top of the stack even while the sheet is moving. The consequence of this may then be damage to the top side of the sheet on top of the stack. This danger is particularly likely when dealing with sheets that have had their surfaces treated as, for example, when the sheets are freshly lacquered. A further undesired effect is that of the sheet curling, as it will be prone to do if it has a tendency to curl due to a coating of lacquer or owing to drying etc. This is therefore a further reason likely to be a source of functional disorders.